Projection-type display device

ABSTRACT

A projection-type display device having a single light source which produces light. A first dichroic mirror group receives the light and separates it into colored light. A transmissive light valve system modulates the colored light and a second dichoric mirror group synthesizes the modulated colored light. A projection system projects the synthesized colored light onto a screen.

This is a division, of application Ser. No. 06/786,438, filed on Oct.11, 1985.

BACKGROUND OF THE INVENTION

The present invention generally relates to a projection-type colordisplay device and, in particular, to a projection-type color displaydevice which uses a plurality of light valves for forming pictureimages.

There are several versions of projection-type color display deviceswhich utilize a light valve. For example, U.S. Pat. Nos. 4,461,542 and4,425,028 disclose a color display device in which a reflecting lightvalve and dichroic mirrors synthesize and project monochromatic picturesA color display device which uses an oil-membrane light valve isdisclosed in an article entitled "Matrix-Addressed Liquid CrystalProjection Display" published in the 1972 Society for InformationDisplay (SID) at pp. 62-63, and in an article entitled "Recent Advancesin the Single-Gun Color Television Light-Valve System" published in the1975 SID at pp. 24-27. An article on the relevant technology entitled"Optical Properties of a Liquid-Crystal Image Transducer at NormalIncidence: Mathematical Analysis and Application to the Off-State" canbe found in J.Opt.Soc.Am., Vol 70, No. 3, March 1980 beginning at page287.

The conventional projection-type color display devices have severaldeficiencies. In a device using a reflecting light valve: first, thereflected light at the surface of the light valve causes deteriorationof the contrast of the displayed images; second, since the light valveis addressed by the light from a cathode ray tube (CRT), the deviceinevitably becomes large and complex, and; third, there is therequirement for an excellent polarized light dividing characteristic aswell as a colored light dividing characteristic of the dichroic mirror.

In the second device using an oil-membrane light valve, the device islarge, complex and expensive and is less than satisfactory with respectto its life span and utilizing efficiency of the light.

Accordingly, the present invention was developed to eliminate theproblems in the prior art as described above and to provide asmall-sized projection-type color display device which is excellent inthe contrast of the displayed pictures and in the utilizing efficiencyof the light from the light source.

SUMMARY OF THE INVENTION

Generally speaking, in accordance with the present invention, aprojection-type display device is provided. The display device includesa first dichroic mirror group which divides light from a light sourceinto a transmitted light and a relected light according to thewavelength thereof. The device also includes transmissive light valveswhich develop the picture images and which modulates the divided lightflux, a second dichroic mirror group for synthesizing the light fluxtransmitted through the light valve and a projection optical system forprojecting the synthesized light flux.

The light length selecting characteristic of the first and seconddichroic mirror groups may be almost equal so that the light fluxseparated by the first dichroic mirror group is synthesized by thesecond dichroic mirror group in the present invention. Moreover, each ofthe first and the second dichroic mirror groups may consist of aplurality of mirrors which effect segregation and synthesis of theprimary colors of red, blue and green.

The first and the second dichroic mirror groups may also have adifferent wavelength selecting characteristic from each other and may bearranged in a cross in one common plane. Moreover, in accordance withthe present invention, the transmissive light valves may have a maximumextinction ratio with respect to the main wavelength of the coloredlight passing therethrough. Furthermore the ray transmitting directionof the light valves for the passing colored light may be inclined withrespect to the normal line of the surface plane of the light valve,thereby most effectively utilizing the incident light angle dependencyof the extinction ratio.

Accordingly, it is an object of the present invention to provide animproved projection-type display device.

Another object of the present invention to provide a projection-typedisplay device which utilizes the properties of a light valve to projectimages.

Yet another object of the present invention is to provide aprojection-type display device which is small in size.

A further object of the present invention is to provide aprojection-type color display device which provides an excellentcontrast in the pictures displayed.

A still further object of the present invention is to provide aprojection-type color display device which has an excellent utilizingefficiency of the light produced by the light source.

Still other objects and advantages of the invention will in part beobvious and will in part be apparent from the specification.

The invention accordingly comprises the features of construction,combination of elements, and arrangement of parts which will beexemplified in the constructions hereinafter set forth, and the scope ofthe invention will be indicated in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the invention, reference is had to thefollowing description take in connection with the accompanying drawings,in which:

FIG. 1 is a diagramatic representation of an illuminating system of afull-color projection-type display device constructed in accordance withthe present invention;

FIG. 2 is a schematic diagram depicting the structure of aprojection-type display device in which the illuminating system of FIG.1 is utilized;

FIG. 3 is a schematic diagram similar to FIG. 2 in which an inclinedlight valve is utilized;

FIG. 4 is a schematic diagram which depicts the plane-arrangementstructure of the display device constructed in accordance with analternative embodiment of the present invention;

FIG. 5 is a schematic view which illustrates a cross dichroic mirrorplane-arrangement structure of a display device constructed inaccordance with another alternative embodiment of the present invention;and,

FIG. 6 is a schematic circuit diagram for explaining the driving methodof the light valve used in the present invention.

FIG. 7 is a graph depicting the wavelength dependency in the guest-hostmode.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The principle of the function of the present invention is firstdescribed. Thereafter, specific embodiments will be described withreference to the drawings.

Light emitted from a light source is segregated into a plurality ofcolored lights by a first dichroic mirror group. Thereafter, atransmissive light valve corresponding to the colored light develops apicture image and the colored lights are picture-modulated. Thetransmissive light valve removes the bad influence of the reflectedlight at the surface of the light valve which is unavoidable in the caseusing a reflection light valve. Accordingly, the transmissive lightvalve improves the contrast of the displayed pictures.

Furthermore, the picture display panel driven by the electro-opticaleffect realizes a motion picture image. The electro-optical materialsuch as liquid crystal, PLZT and the like can be used in the lightvalve. In any case, compared with the reflection light valve in whichthe image is written by a light beam from a CRT, and an oil-membranelight valve, the use of the transmissive light valve which is thin andsmall provides more freedom of construction of the display device andmakes it possible to obtain a small display device.

The picture-modulated colored lights are synthesized by a seconddichroic mirror group. The second dichroic mirror group may be of thesame wavelength selecting characteristic as the first dichroic mirrorgroup, so that the second dichroic mirror group synthesizes theseparated colored lights reversibly.

For example, for the first dichroic mirror group for separating a whitecolor light into red, green and blue lights, the corresponding seconddichroic mirror group has a red, green and blue light segregatingcharacteristic almost equal to that of the first dichroic mirror groupand synthesises the red, green and blue colored lights including thepicture information of the corresponding color (hereinafter referred toas the colored picture lights) reversibly. If all of the used lightvalves are transmissive, the synthesized light is the same color as thelight from the light source. In the present invention, the onlynecessary ability of the dichroic mirror is to segregate the light intothe colored lights. The ability to separate the polarized lightcomponent, which is required in the prior art, is not necessary in thepresent invention.

As described above, since the light from the light source is separatedinto colored lights and the separated colored lights are modulated andsynthesized, there is no need to provide a plurality of light sourcescorresponding to the light of each color. One single light source issufficient for use in the present invention.

Next, the synthesized colored picture light is focused into an imagethrough a projection lens onto a screen. Since a plurality of coloredlights are synthesized, the picture corresponding to each color must beaccurately positioned. For example, in the case of a full-color displayusing the primary colors of red, green and blue, pictures of eachprimary color are synthesized while insuring good convergence.

By arranging the first and the second dichroic mirror groups having thedifferent colored light dividing characteristics with each other in across arrangement, the length of the light which passes between thelight source and the light valve, and between the light valve to theprojecting optical system, are shortened. Also, by providing thedichroic mirror groups in one common plane, a thin and small-sizedprojection-type display device is obtained.

In the present invention, the transmissive light valve may have awavelength dependency of the extinction ratio which coincides with themain wavelength of the colored light passing therethrough. This improvesthe contrast of the picture images obtained by synthesizing andprojecting the colored light through the light valve. For example, whereusing the twisted nematic liquid crystal mode (hereinafter referred toas "TN liquid crystal mode"), the peak of the wavelength-lighttransmittance curve due to the retardance thereof is made to coincidewith the main wavelength of each colored light. As another example,where using the guest-host liquid crystal mode, the peak of thedichroism ratio of the dichroic pigment is made to coincide with themain wavelength of each colored light.

Table 1 shows the wavelength dependency of the mode of the light valveIn this invention, the light valve may be constructed so that theparameters such as thickness and pigment used are changed such that themain wavelength of each colored light is appropriate for the wavelengthdependency indicated in Table 1.

                  TABLE 1                                                         ______________________________________                                        Light Valve Mode                                                                          Wave Length Dependency                                            ______________________________________                                        TN Liquid Crystal                                                                         Birefringence of Liquid Crystal (Δn)                        Mode                                                                                       ##STR1##                                                                      ##STR2##                                                                     T: transmittance λ: wave length d: thickness               Birefringence                                                                             Birefringence of Material (Δn)                              Mode                                                                                       ##STR3##                                                                     K: constant                                                       Guest-Host  Absorption Characteristic of Dichroic                             Mode        Pigment see FIG. 7.                                               Color Polarizer                                                                           Polarizing-Wave Length Characteristic of                          TN Liquid Crystal                                                                         Color Polarizer                                                   Mode                                                                          ______________________________________                                    

Furthermore, in the present invention, the transmissive light valve mayhave a ray transmitting direction of the colored light passingtherethrough which inclines by an angle of between 0° to 45° withrespect to the normal line of the surface plane of the light valve.Namely, the light valve surface may be inclined so as to increase theextinction ratio in view of the incident light angle dependency of theextinction ratio of the transmissive light valve. Examples of the lightvalve modes having the incident light angle dependency are the TN liquidcrystal mode, the guest-host mode and birefringence mode in Table 1.

The present invention is now explained in detail with reference to thedrawings. Reference is first made to FIG. 1 which depicts anilluminating structure generally indicated at 30 of full-colorprojection-type display device, constructed in accordance with thepresent invention. In FIG. 1, a dichroic mirror group for reflectingblue light (B mirror) 1 and a dichroic mirror group for reflecting redlight (R mirror) 2 are arranged in a cross arrangement for performingthe segregation and the synthesis of an incident light flux 8. Mirror 3changes the direction of the light flux, and a transmissive light valve4 develops the picture images corresponding to red, green and blue. Inthis embodiment, the liquid crystal panel of the active-matrix (such asa thin film transistor (TFT) matrix) driving method is used as the lightvalve 4.

Reference is now also made to FIG. 2 in which a display device generallyindicated at 40 including the projection optical system of FIG. 1 isschematically depicted. FIG. 2 shows only a green light beam segregationfor simplification. As an illuminating system, Kohler illumination, acritical illumination, a telecentric illumination or the like may beused. The system includes a condenser lens 5, a projection lens 6, alight source 7 and a screen 9.

The principle of the device in accordance with the present invention isexplained with reference to FIGS. 1 and 2. As shown, light source 7 (forexample, a halogen lamp) emittes a white light which is condensed bycondenser lens 5. White light 8 which enters into dichroic mirror groups1 and 2 are separated into the red (R), green (G) and blue (B) lights,the direction of which is changed by mirror 3 and the colored lightsenter into transmissive light valve 4. The surface of light valve 4 isdereflection-coated so as to effecitvely transmit the incident light.

Light valve 4 is positioned so that the images are focused throughprojection lens 6 onto screen 9 and develops the picture imagescorresponding to each colored light. In this embodiment, the videosignals of red, green and blue (shown as 18 in the circuit or FIG. 6)are delivered to each liquid crystal panel to form the monochromaticdynamic picture images.

In the present embodiment, a liquid crystal panel of the TN liquidcrystal mode is used. According to the wavelength dependency as shown inTable 1 above, the nematic liquid crystal material or Δn=0.15 is usedand the thicknesses of the liquid crystal layers or the red, green andblue light valves are defined to 8.4 μm, 7.1 μm and 5.8 μm,respectively, to make the main wavelength of each colored light coincidewith the second peak of the wavelength-light transmittancecharacteristic of the liquid crystal material. Herein, the thickness ofeach liquid crystal layer is defined by taking into account the constanttemperature after the projection light is applied to the panel.

Since the TN liquid crystal mode presents the incident light angledependency of the extinction ratio, it is effective to provide the lightvalve so that the direction of the incident light inclines with respectto the normal line to the surface plane of the light valve. However, insuch a case as this, as the light deviates from the optical axis of theprojecting lens, the images may be focused on the position whichdeviates from the optical axis and focused in a trapezoidal shape. Theangle of the inclination of the light valve is determined according tothe physical properties and the focusing range of the projecting lens.For the liquid crystal panel of the TN liquid crystal mode used in thisembodiment, the practical angle by which the light valve is inclinedshould be between 0° and 30°. Such an arrangement is shown in FIG. 3.

Reference is now made to FIG. 6 which will be used r explain the drivingmethod of the TFT liquid crystal panel used a transmissive light valve 4in FIG. 2.

Since alternating current driving is required for the liquid crystalpanel, the phase of the video signal 18 is inverted in every other field(F) by a polarity inverter circuit 10. A synchronizing controllercircuit consists of a voltage controlled oscillator (VCO) 11, a loopfilter 12, a phase comparater 13 and divider 14, and produces clock anddata signals X and Y and the one-field (1F) signal.

An X-side shift register 15, a transmission gate 17 for delivering thevideo signals to each picture element and a Y-side shift register 16 arecoupled to liquid crystal panel 4. The X-side and the Y-side shiftregisters address the thin film transistors i the row direction and thecolumn direction, respectively.

By such a structure as above, the picture element voltage correspondingto the video signal is applied to the TFT liquid crystal picture elementto realize the picture display. The detail of the driving method and theliquid crystal panel utilized are i accordance with the disclosure ofNikkei Electronics No. 351, 1984 p 221 and SID '83 Digest, p 156, whichare incorporated by reference herein as though fully set forth.

The liquid crystal panels of each color are positioned so that thedisplayed pictures coincide on the screen. When the primary colors ofred, green and blue are synthesized and the full-color display is to beeffected, misconvergence causes color aberration and a color-ghost.Especially when using the matrix panel, the positions of the panelsshould be determined in a scale less than the pitch between the pictureelements which make up such panels in a liquid crystal panel. The"pitch" is the fixed distance between adjacent picture elements. Hence,the positioning of each of the liquid crystal panels should be adjustedwith respect to one another so that color images from each pictureelement in each of the panels are superimposed on or synthesized withone another equal to or less than the distance equivalent to the pitchbetween adjacent picture element. When the pitch between the pictureelements of each color is set, by regularly shifting each panel about adistance equal to a half pitch of the picture elements, the resolutionof the panel is enhanced more than the monochromatic panel.

As is apparent from FIG. 1, the R panel image and the B panel image arein the relation of mirror images with respect to the G panel image.

In the present invention, the dichroic mirror only has to function tosegregate and synthesize the colored light. However, the reflected lighton the dielectric material thin film always includes the polarizingeffect Namely, in FIG. 1, the red and the blue lights contain morevertically polarized components and the green light contains morehorizontally polarized components. Accordingly, in the electro-opticaleffect mode using a polarizer, it is sometimes necessary to adjust thedirection of the polarizer. For example, in case where TN (nematicliquid crystal twisted by 90°) liquid crystal display mode is used, inorder to use the light flux most efficiently, the polarizers should bearranged so that the transmission axes of the polarizers on the side ofthe entry of light of the R panel and the B panel are vertical and thetransmission axis of the polarizers on the side of the entry of light ofthe G panel is horizontal in FIG. 1.

Moreover, by determining the direction of the polarizers, the whitebalance, that is, the strength of each color is adjusted.

The colored light picture-modulated by the transmissive light valveagain enters into the dichroic mirror group and the red and green andblue lights are reversibly synthesized. Finally, the red and green andblue lights are projected and focused in an image on the screen 9 asshown in FIGS. 1 and 2.

The arrangement of the dichroic mirror group is not limited to the oneshown by FIG. 1. Other construction examples are shown in FIGS. 4 and 5.In the arrangements of FIGS. 4 and 5, the optical centers of the opticalelements including the dichroic mirror groups, light valves and theprojecting optical system are positioned in one common plane, such thatthe principal light rays passing through the optical elements arelocated in a common plane, thereby providing thinner devices than thearrangement of FIG. 1.

The arrangement of. FIG. 4 is simple in which there is n need to providethe dichroic mirror groups 1 and 2 in a cross. Moreover, mirror 3 forrefracting the colored light is also provided on the same plane as thedichroic mirror groups. As in FIG. 1, the system includes a condenserlens 5, a light valve 4 and a projecting lens 6.

FIG. 5 shows an example in which the cross-arranged dichroic mirrorgroups are provided in a plane. The feature of the arrangement of FIG. 5is the length of the optical path between light valve 4 and light source7 and condenser lens 5 is different with respect to each of the red, theblue and the green lights. Furthermore, because the red light and theblue light contain more components which are polarized vertically withrespect to the paper surface, the reflection efficiency at mirror 3 forchanging the direction of the light is improved.

Light valves 4 corresponding to each colored light in FIGS. 4 and 5 arepositioned, similarly to FIG. 1, so that the light valves for all colorsare at the focusing position which is at the optically equal distancefrom projecting lens 6.

As shown by FIGS. 2, 3, 4 and 5, in accordance with the presentinvention, only a single projecting lens is required and there is noneed to adjust the convergence between the picture images of each colorwhen changing the projecting magnification or projecting distance.

In the above description, the TN liquid crystal panel using TFT is usedas the transmissive light valve, for example. However, the light valvemodes shown in Table 1 can of course be used. Moreover, other lightvalve modes than those in Table 1 such as the switching phenomenon froma scattering state to a transparent state (the dymanic scattering modeof liquid crystal, etc.), the storing display mode of the liquid crystaland the like may also be applied. Furthermore, the applicable materialis not limited to a liquid crystal material, but as long as the materialis transmissive, other materials such as of the electro-optical effectof the light-transmissive ceramic including PLZT, electrochromic,electrophoretic and the like may also be used.

Also, in the above embodiment, the primary colors of red, green and blueare separately synthesized. However, other numbers of colors such as 2,or more than 3, are acceptable.

As explained, by using the dichroic mirror groups having a wavelengthselecting characteristic which is almost the same as that of thetransmissive light valve, the deterioration of the contrast of thedisplay and the color reproductivity caused by the reflected light areavoided and a small-size projection-type display device is provided inaccordance with this invention. Moreover, because of the excellentcolored light separating ability of the dichroic mirrors, the projectionof the images is realized by a single light source and the light flux isutilized very efficiently. In addition, since one single projection lensis sufficient, the magnification is easily varied. The device of thepresent invention is further advantageous in that the life of the deviceis considerably longer without requiring maintenance compared with theconventional high-light flux projecting display device. Also, bysufficiently utilizing the extinction property of the light valve, thepictures of the high contrast and excellent color reproductivity aredisplayed.

It will thus be seen that the objects set forth above, among those madeapparent from the preceding description, are efficiently attained and,since certain changes may be made in the above construction withoutdeparting from the spirit and scope of the invention, it is intendedthat all matter contained in the above description or shown in theaccompanying drawings shall be interpreted as illustrative and not in alimiting sense.

It is also to be understood that the following claims are intended tocover all of the generic and specific features of the invention hereindescribed and all statements of the scope of the invention which, as amatter of language, might be said to fall therebetween.

What is claimed is:
 1. A projection-type display device comprising alight source for producing light, color segregating means for receivingsaid light and segregating said light into at least two differentcolored lights, color modulating means having at least two light valveseach of which receives one of said colored lights, modulates saidcolored lights and generates color images in response thereto, colorsynthesizing means including at least two dichroic optical elements forsynthesizing said color images, and projection means for projecting saidsynthesized image, each modulated colored light having a polarizationaxis, said light incident to and said light outputted from said colorsynthesizing means each having an optical axis, said polarization axesof said modulated color lights being substantially perpendicular to aplane formed by the optical axis of the light incident to said colorsynthesizing means and the optical axis of the light outputted from saidcolor synthesizing means when said colored lights are reflected at leastonce at said dichroic optical elements, the polarization axes of saidmodulated color lights being substantially parallel to said plane whensaid colored lights are transmitted through said dichroic opticalelements.
 2. The projection-type display device as claimed in claim 1,wherein said color modulating means includes first, second and thirdliquid crystal light valves each having a polarizer means, thepolarization axes of the polarizer means being adjusted so that thecolored lights modulated by said first and second light valves arepolarized lights having polarization axes which are perpendicular tosaid plane, the colored light modulated by said third light valve beinga polarized light having a polarization axis which is parallel to saidplane.
 3. The projection-type display device as claimed in claim 2,wherein said color segregating means segregates red, blue and greencolored lights, said first liquid crystal light valve modulating saidred colored light, said second liquid crystal light valve modulatingsaid blue colored light and said third liquid crystal light valvemodulating said green colored light.
 4. The projection-type displaydevice as claimed in claim 2, wherein the white balance defined by theintensity of each color is selected by adjusting the direction of thepolarizers associated with each light valve.
 5. The projection-typedisplay device as in claim 1, wherein each of said at least two dichroicoptical elements synthesize at least two different colored lights into asingle light beam.